1. Field of the Invention
The invention relates in general to a method for fabricating a circuit substrate, and more particularly to a method for fabricating a conductive blind via of a circuit substrate.
2. Description of the Related Art
Circuit substrates are formed by stacking several dielectric layers and several patterned circuit layers alternately. The patterned circuit layers are defined by lithography and etching a copper layer. The dielectric layers are disposed between the patterned circuit layers for protecting and separating the patterned circuit layers. The patterned circuit layers are electrically connected to each other through conductive blind vias in the dielectric layers. Furthermore, several contacts are formed on surfaces of the circuit substrates for electrically connecting with an outer electronic device. In the present conductor packaging technology, circuit substrates have become the main trend of chip packaging because of the advantages including fine wiring structures and good quality.
FIG. 1 illustrates a conventional circuit substrate. Please referring to FIG. 1, a conventional circuit substrate 100 includes a core layer 110, a patterned circuit layer 120, a dielectric layer 130 and at least a conductive blind via 140. The patterned circuit layer 120 disposed between the core layer 110 and the dielectric layer 130 includes at least a capture pad 122. The capture pad 122 is made of copper. The conductive blind via 140 penetrates the dielectric layer 130 and contacts the capture pad 122.
In the conventional circuit substrate 100, the method for fabricating the conductive blind via 140 includes following steps. First, a blind via 142 penetrating the dielectric layer 130 is formed by laser drilling. The blind via 142 exposes the capture pad 122. Next, an electroless copper layer 144 (composed of sub-micro level copper particles) is formed on the capture pad 122 and an inner wall of the blind via 142 through an electroless plating process. Then, an electroplated copper layer 146 is formed on the electroless copper layer 144 by an electroplating process. The entire blind via 142 is filled with the electroplated copper layer 146. As a result, the conductive blind via 140 is formed by the above steps.
Although the capture pad, the electroless copper layer and the electroplated copper layer are all made of copper in the above circuit substrate, all of them have different microstructures. The electroless copper layer made of sub-micro level copper particles has less strength. Therefore, a portion of the electroless copper layer contacting the capture pad flakes off easily due to thermal stress generated when the circuit substrate is functioning. A bad contact occurs between the conductive blind via and the patterned circuit layer. The electrical connection between the conductive blind via and the patterned circuit layer fails easily. Therefore, reliability of the circuit substrate is lowered.